A comprehensive mathematical model of chlorine etching of polysilicon in a single-wafer plasma reactor was developed from first principles. Wafer heat transport was included as an integral part of the analysis. The model provided the spatiotemporal variations of etchant concentration, wafer temperature, and etch rate. Under conditions of high etch rate and poor wafer cooling, etching was inherently transient. The polysilicon etch rate increased with time despite the fact that the atomic chlorine concentration decreased with time. This was due to wafer heating and the Arrhenius dependence of etch rate on temperature. Multichannel laser interferometry was developed to monitor the spatiotemporal variations of etch rate in real time. Measured etch rate transients compared favorably with model predictions. Results were sensitive to surface reaction parameters and to wafer back side cooling.